Corals reveal ENSO-driven synchrony of climate impacts on both terrestrial and marine ecosystems in northern Borneo

Extreme climate events, such as the El Niños in 1997/1998 and 2015/16, have led to considerable forest loss in the Southeast Asian region following unprecedented drought and wildfires. In Borneo, the effects of extreme climate events have been exacerbated by rapid urbanization, accelerated deforestation and soil erosion since the 1980s. However, studies quantifying the impact of interannual and long-term (>3 decades) climatic and anthropogenic change affecting Borneo’s coastal and coral reef environments are lacking. Here, we used coral cores collected in Miri-Sibuti Coral Reefs National Park, Sarawak (Malaysia) to reconstruct the spatio-temporal dynamics of sea surface temperature and oxygen isotopic composition of seawater from 1982 to 2016, based on paired oxygen isotope and Sr/Ca measurements. The results revealed rising sea surface temperatures of 0.26 ± 0.04 °C per decade since 1982. Reconstructed δ18Osw displayed positive excursion during major El Niño events of 1983, 1997/98 and 2015/16, indicating drought conditions with less river runoff, rainfall and higher ocean salinities. La Niñas were generally associated with lower δ18Osw. We observed a long-term shift from more saline conditions between 1982 and 1995 towards less saline conditions after 1995, which are in agreement with the regional freshening trend, punctuated by saline excursion during El Niños. The decadal shifts were found to be driven by the Pacific Decadal Oscillation (PDO). This study provides the first long-term data on El Niño Southern Oscillation (ENSO)-driven synchrony of climate impacts on both terrestrial and marine ecosystems in northern Borneo. Our results suggest that coral records from northern Borneo are invaluable archives to detect regional ENSO and PDO impacts, and their interaction with the Asian-Australian monsoon, on the hydrological balance in the southern South China Sea beyond the past three decades.

In northern Borneo, natural climatic events paired with currently unquantified contributions from climate change may exacerbate the regional anthropogenic impacts on coral reefs. The regional climate is strongly influenced by the Asian-Australian monsoon system [13][14][15] . The winter monsoon usually lasts from November to March, characterized by strong northeasterly winds, colder sea surface temperatures (SSTs), wetter conditions and lower salinities ( Fig. 1; see Supplementary Fig. S1). The summer monsoon, usually from May to September, is distinguished by reduced southwesterly winds, highest SST, lower precipitation and higher salinity ( Fig. 1; see Supplementary Fig. S1). However, precipitation in northern Borneo is more evenly distributed across the annual cycle, less monsoonal and dominated by intraseasonal and interannual variability 16 . Intra-monsoon

Results
Monthly interpolated geochemical proxy time series. The monthly interpolated time series of AG and EG coral Sr/Ca and δ 18 O revealed a distinct seasonality with higher values during the winter season (December to February) and lower values in the summer season (June to August) (Fig. 3). Spatial comparisons of the two reefs between 2006 and 2016 found no difference between Sr/Ca ratios at AG and EG (see Supplementary  Table S1). For mean coral δ 18 O, AG had lower values than EG ( Fig. 3; see Supplementary Table S1). Linear correlations between Sr/Ca and δ 18 O in both AG and EG were highly significant (p < 0.0001). The correlation between Sr/Ca and δ 18 O at AG (r = 0.81) was stronger than at EG (r = 0.62).
AG Sr/Ca ratios displayed an overall decrease of 0.06 mmol/mol from 1982 to 2016. Seasonal Sr/Ca amplitudes also decreased at AG from approximately 1993, although there were higher winter values in some years (e.g. 2014 to 2016; Fig. 3a). There was no trend in Sr/Ca at EG between 2006 and 2016. The AG δ 18 O record showed an overall decrease of 0.45‰ and had stronger interannual and decadal variability than the Sr/Ca record (Fig. 3b). In the long term, the annual mean δ 18  Monthly calibration of Sr/Ca with sea surface temperature (SST) and SST variability. Ordinary least squares regressions between coral Sr/Ca and AVHRR-OISSTv2 46,47 were used to reconstruct absolute SST from coral Sr/Ca (Table 2). Our observed strong correlation between Sr/Ca and SST at the study sites indicates that growth rate and vital effects had an insignificant effect on skeletal Sr/Ca. The local proxy-SST relationships varied between −0.043 mmol/mol °C −1 in AG and −0.039 mmol/mol °C −1 in EG, which is in agreement with the range of published slopes 26,29 that lie between −0.04 and −0.08 mmol/mol °C −1 . Our regression slopes were also comparable to other coral core studies in the northern SCS [48][49][50] . The root mean square error (RMSE) of individual monthly interpolated reconstructed SST for the AG and EG record was 0.64 °C and 0.55 °C, respectively.
Reconstructed monthly interpolated SST across the two sites varied between 26.5 °C and 30.5 °C in Sr/ Ca-SST and between 25.9 and 30.8 °C in AVHRR-OISSTv2 data for the 0.25° × 0.25° grid 46,47 . Both Sr/ Ca-SST (0.85 ± 0.13 °C) and instrumental SST (0.41 ± 0.18 °C) showed a long-term warming trend (Fig. 4a). AVHRR-OISSTv2 had larger seasonal amplitudes, especially before 2005, with lower winter SSTs and higher   summer SSTs (Fig. 4a). Within the reconstructed SST data, seasonal amplitudes declined after 1993, with a shift to higher winter temperatures and smaller seasonal amplitudes (see Supplementary Fig. S2). Occasionally, winter Sr/Ca-SSTs reached values similar to pre-1993, for instance between 2002-2003 and 2014-2016 (Fig. 4b). The coral-derived SST anomalies in both AG and EG relative to the 2006 to 2016 mean (years of coral core overlap) mirrored the interannual variations of instrumental SST anomalies well (Fig. 4b). Both AVHRR-OISSTv2 and our coral Sr/Ca-SST records showed no significant correlation with the Niño3.4 index, yet both the AVHRR-OISSTv2 and AG Sr/Ca-SST had significant relationships with Niño4 in Boreal winter to spring (see Supplementary  Table S3). AG Sr/Ca anomalies relative to a 1982 to 2016 mean were converted to SST using the local calibration slope (−0.04 mmol/mol °C −1 ; Fig. 4b), a mean slope of −0.06 mmol/mol °C −1 and a slope of −0.08 mmol/mol °C −1 to estimate an error range of the calibration 26,29 . The coral-derived SST anomalies followed the trends of instrumental SST anomalies reasonably well (Fig. 4b). However, the regression slopes of −0.08 mmol/mol °C −1 and −0.06 mmol/mol °C −1 resulted in SST anomalies that closely aligned with AVHRR-OISSTv2, while the locally derived −0.04 mmol/mol°C −1 slope estimate deviated the most. Between 1982 and 1994, the AG Sr/Ca-SST anomalies (relative to the 1982-2016 mean) were 0.5 °C lower than between 1994 and 2016. Sr   Table S1). Mean annual and detrended δ 18 O sw in AG and EG were highly correlated (r = 0.93, p < 0.001). There was also a positive and significant correlation between mean annual reconstructed δ 18 O sw and EN4 SSS at AG (r = 0.72, p < 0.001) and EG (r = 0.68, p = 0.046). The correlation remained significant for seasonal mean values (see Supplementary Table S4). δ 18 O sw in AG and EG mostly mirrored the long-term and interannual to decadal variability of the EN4 SSS data between 1982 and 2016, with higher values matching higher salinity (Fig. 5a). EG δ 18 O sw values showed a higher variability in individual years than AG between 2006 and 2016. AG δ 18 O sw between 1982 and 2016 indicated a decrease of 0.27 ± 0.04‰ (p < 0.001), while EN4 SSS decreased by 0.32 ± 0.06 psu (p < 0.001). Both AG and EG δ 18 O sw displayed decadal variability between 2006 and 2016 in agreement with EN4 SSS (Fig. 5a). Mean annual δ 18 O sw at AG and EG, and EN4 SSS were negatively correlated with the local precipitation and Marudi river discharge. All correlations were statistically significant with the exception of EN4 SSS with Marudi discharge (Table 3).  www.nature.com/scientificreports www.nature.com/scientificreports/ Anomalies of reconstructed δ 18 O sw and EN4 SSS were calculated relative to the 2006 to 2016 means (Fig. 5b). With some exceptions, AG's δ 18 O sw followed the interannual variability and trends observed in EN4 SSS since 1982 (Fig. 5b). The comparison of the anomalies with ENSO indices Niño3.4 and Niño4 51 showed good agreement between the coral-derived δ 18 O sw (AG and EG) and ENSO indices. For example, high δ 18 O sw matched the 1997/98 El Niño and was followed by a 1998/99 La Niña phase and a shift to low δ 18 O sw in the coral record ( Fig. 5b; see Supplementary Fig. S2). Generally, both AG and EG recorded similar interannual variability with slightly larger magnitudes for EG (Fig. 5b). Both AG and EG δ 18 O sw also largely mirrored interannual variations in river discharge and precipitation anomalies (Fig. 5c,d). Marudi discharge indicated a significant increase of 641 ± 121 m 3 /s between 1989 and 2015 (p < 0.001) while Miri precipitation recorded a non-significant increase of 56 ± 38 mm month −1 (p = 0.15) between 1992 and 2016.
Regional and large-scale teleconnection of reconstructed δ 18 o sw with climate parameters. Coral δ 18 O sw at AG and EG, precipitation from the local weather station, Marudi river discharge and EN4 SSS were correlated with ENSO SST indices Niño3.4 and Niño4 51 and the PDO 52 (Table 4). All correlations with ENSO indices, exhibited statistical significance averaged over a 3-month period ( Table 4). All data, except for precipitation and river discharge (negative correlation), were significantly positively correlated with Niño3.4 and Niño4 (Table 4). Miri precipitation and AG δ 18 O sw had the highest correlations in the February to April (FMA) season, while EN4 SSS had it between March to May (MAM) ( Table 4). EG δ 18 O sw showed the highest correlations with Niño3.4 and Niño4 between November to January and December to February, respectively (Table 4). Miri precipitation and Marudi river discharge indicated the highest, statistically significant (p < 0.05) correlations between February and April and August to October (Table 4).  Table 4). The same holds for mean annual δ 18 O sw at AG (r = 0.56, p = 0.002) and EG (r = 0.75, p = 0.02). Marudi discharge  and Miri station precipitation (1992-2016) were significantly negatively correlated with the PDO on seasonal time scales (February to April; Table 4). The correlation was positive and lower between EN4 SSS and the PDO, yet significant (p < 0.05) for both seasonal (Table 4) and mean annual time scales (r = 0.38, p = 0.038).
Spatial correlations of AG and EG δ 18 O sw and local precipitation against OISSTv2 displayed the typical ENSO "horseshoe" SST pattern at both seasonal and mean annual time scales (Fig. 6a-d). AG and EG δ 18 O sw were positively correlated with the central and eastern Pacific SST and negatively correlated with the northwestern, central and southwestern Pacific SST. Further there was a positive correlation with the northeastern and central Indian Ocean SST and negative correlation with the southeastern Indian Ocean SST (Fig. 6a-d).
Correlations of local rainfall (Fig. 6e,f) and river discharge (Fig. 6g,h) indicated the same pattern in the Pacific, except with an inverse relationship between precipitation, river discharge and SST. There was significant negative correlation between Miri precipitation and river discharge with northern and central Indian Ocean SST for both seasonal (FMA) and annual means (Fig. 6e,f). Additionally, seasonal means of local precipitation and river discharge displayed positive correlation with southeast Indian Ocean SST. The spatial correlations of Miri station precipitation with SST also revealed the "horseshoe" ENSO SST pattern (Fig. 6g,h).

Discussion
Our observed strong correlation between Sr/Ca and SST at the study sites indicates that growth rate and vital effects had an insignificant effect on skeletal Sr/Ca. The Sr/Ca-SST reconstructions from the AG coral core record suggested an increase of 0.26 ± 0.04 °C per decade since 1982, with more pronounced warming during the winter    www.nature.com/scientificreports www.nature.com/scientificreports/ 0.16 °C per decade for summer SSTs. The stronger increase in winter SST might be related to a weakening of the East Asian Winter Monsoon (EAWM) 48,55,56 , which results in less transport of cooler extratropical waters from the north through the Taiwan Strait fed by the Western Pacific. Thus, the West Pacific seems to be the dominant factor controlling SST variations in the SCS, especially in the southern section, where the monsoonal influence is generally lower than in the north 48,57 . On interannual time scales, ENSO dominates the SST response in the SCS with warm anomalies in the mature phase of El Niño (December to February) extending into spring (March to May) 58,59 . However, correlations of AVHRR-OISSTv2 near our study sites and our Sr/Ca records with the Nino3.4 index were not significant during the Boreal winter, while those with the Nino.4 index were weak, yet significant between January and April. This indicates that the southern SCS is indeed most sensitive to western Pacific SST and circulation anomalies as proposed by Juneng and Tangang 58 . Decadal changes superimposed on the overall rising temperature trend is a common observation in coral records from the SCS 48,60 . Drivers of these decadal shifts could include the PDO or Interdecadal Pacific Oscillation (IPO 61 ). AVHRR-OISSTv2 did indicate weak, yet significant correlations with the PDO on seasonal timescales (r = 0.41, p = 0.017), yet not on mean annual time scales. Correlations between the PDO and AG and EG Sr/Ca were not significant. Thus, the influence of the PDO on SST in the southern SCS remains uncertain.
Coral-derived δ 18 O sw from AG and EG exhibited significant correlation with ocean SSS (offshore grid), local precipitation and river discharge suggesting that the coral record can provide reliable information on changes in the hydrological balance in reef waters. Regional monitoring of oxygen isotopes in rainfall between 2004 and 2011 upstream and downstream in the Baram catchment (Mulu and Lambir Hills National Park) indicated a clear inverse relationship with the amount of rainfall 62,63 . Therefore, δ 18 O of seawater around Miri is influenced by the balance between precipitation and evaporation (the P-E balance), and freshwater runoff. Thus, sea surface salinity, which is also largely modified by P-E balance and riverine freshwater runoff, is linearly related to δ 18 O sw 39,64,65 . In the southern part of the SCS precipitation exceeds evaporation 57 . EN4 SSS 66 for the grid closest to our coral reefs showed significant correlations with precipitation at Miri (r = −0.53, p < 0.009) yet not with river discharge at Marudi (r = −0.29, p = 0.166). The modest correlations of EN4 SSS 66 with precipitation and river discharge could be explained by salinity being averaged over a larger offshore oceanic grid (3.5-4.5°N, 112.5-113.5°E) that is some distance from the rainfall station and rivers. However, correlations of coral δ 18 O sw from AG and EG with the local precipitation (AG: r = −0.68, EG: r = −0.83, p < 0.003) indicated a significant inverse relationship between the reefs' seawater δ 18 O and the seasonal cycle of local rainfall, with lowest δ 18 O sw during the winter monsoon. The same holds for correlations with river discharge at both EG and AG. Mean δ 18 O sw in EG and AG did not differ, despite EG's closer proximity to the river mouths. Exceptionally lower peaks in EG than AG, for example in the winter months of 2008/9 and 2010/11, could be attributed to heavy rainfall and high runoff events, which are typically more pronounced closer to the river source. The AG δ 18 O sw record for 1982 to 2016 was also in good agreement with trends and interannual to decadal variability in oceanic salinity data with few exceptions. The same holds for correlations between AG δ 18 O sw and river discharge between 1989 and 2016. As such, both AG and EG reliably recorded the regional P-E balance and river discharge. Yet, isotope analysis and salinity measurements of water samples on a transect from the river mouth to the reefs would provide a better understanding of riverine impact in future studies.
Both the gridded SSS data and the coral-derived δ 18 O sw displayed an interdecadal shift in the mid-1990s superimposed on the interannual ENSO variability (see Supplementary Fig. S3). Several studies in the SCS attributed these interdecadal shifts to the PDO. Zeng et al. 67  ). Both AG and EG δ 18 O sw, as well as precipitation and river discharge, indicated robust relationships with the PDO, while EN4 SSS showed weaker, yet statistically significant relationships (Table 4). It has been suggested that the positive PDO phase induces dry sinking air along with reduced rainfall and vice versa in the negative phase in the SCS 23 . In addition, salinification during positive PDO is further enhanced due to a directional switch in the horizontal current through the Luzon Strait, leading to the advection of more saline waters into the SCS 67 . In line with these findings, Deng et al. 68 showed significant correlation between δ 13 C and δ 18 O from a coral proxy record in the northern SCS and the PDO. They suggest that PDO is causing precipitation anomalies on interdecadal scales, influencing the hydrological balance in the region due to changing rainfall and river runoff dynamics. It is therefore likely that Pacific decadal variability has influenced long-term salinity trends in the SCS. Therefore, our coral δ 18 O sw records should be extended in future studies to cover the past two centuries to assess the stability of the ENSO teleconnection and decadal shifts related to the PDO identified in our study.
The correlations of precipitation and the coral δ 18 O sw in Miri with ENSO indices demonstrated a robust relationship between the ENSO phenomenon and hydroclimate in northern Borneo. The spatial correlations resembled the ENSO "horseshoe" SST pattern in agreement with previous studies 19,58 (Fig. 6). The El Niño events in 1983,1988,1992,2010 and 2014-2016 all showed consistent positive δ 18 O sw and salinity anomalies indicating reduced runoff or P-E testifying to the paramount impact of ENSO on SEA drought. The Marudi river discharge and Miri precipitation records confirmed the reduction in freshwater runoff from the Baram River since 1989 and precipitation since 1992 during all El Niño years. The comparison of coral-derived δ 18 O sw anomalies with the SST anomalies in the Niño 3.4 and Niño 4 regions also displayed good agreement, with a particularly striking shift in δ 18 O sw anomalies matching the 1997/98 El Niño followed by 1998/99 La Niña. The latter is not surprising since winter precipitation in Borneo was reduced by more than 50% during the 1997/98 El Niño event 69 1996,[1999][2000][2001]2008 and 2011 negative δ 18 O sw were observed in agreement with lower SSS and higher river discharge. Several other paleoclimate studies in the SCS on corals from the northern SCS 48 , a speleothem record from northwestern Borneo 70 and the δ 18 O of rainfall interannual anomalies at Gunung Mulu and Lambir Hills National Parks in northern Borneo have also shown clear ENSO impacts 62,63 . The proxy records together with the rainfall isotope data support the observed pattern of anomalously low precipitation and droughts in the Maritime Continent during El Niño 71,72 . This drying across the Maritime Continent is associated with warm SST anomalies during the El Niño mature phase causing an eastward shift in convection 72 . During La Niña convection strengthens in the Maritime Continent, causing anomalously high precipitation and floods. In northern Borneo the impact of ENSO starts in Boreal autumn and persists through Boreal spring with strongest responses in the winter season 19,58,73,74 , which is in accordance with the results from the coral records. The peak drying response to El Niño in northern Borneo in winter and spring is associated with the northeastward shift in intraseasonal convective activity into the northern hemisphere and the development of an anticyclone in the West North Pacific 19,58 . Tangang and Juneng 19 suggested that the correlation between northern Borneo precipitation and ENSO in December to February is not a direct response to the eastern Pacific pole of canonical ENSO, but rather related to a dipole in SST strengthening the western Pacific arm of the 'horseshoe' SST pattern generated by ENSO. This interpretation is in agreement with our results showing highest correlations of coral δ 18 O sw (also precipitation, Marudi discharge and Miri precipitation) in winter/early spring (February to April) with northwestern Pacific SST east of the Philippines (Fig. 6). Thus, coral δ 18 O sw proves to be a reliable tracer of ENSO impacts on precipitation and river discharge in the region. In addition, our results provided clear evidence for ENSO-driven synchrony of climate impacts on both terrestrial and marine environments in northern Borneo. Furthermore, our findings provided invaluable data to better understand ENSO and PDO impacts on the southern SCS, but also for reef monitoring and protection, since both thermal stress and freshwater flux and related sediment transport may put coral reefs increasingly at risk along the northern Borneo coast in the coming decades. Future studies will expand the proxy reconstructions for this region both temporally and spatially to test for long-term stability of ENSO and PDO relationships with the Asian-Australian monsoon and its potential modification by recent global warming.

Methods
Coral sampling and core treatment. In September 2016, coral cores were retrieved from two reef sites in MSCRNP, at Sarawak's northern coast (Fig. 1). Fieldwork was approved by the Sarawak Forestry Commission (permit no (61)/JHS/NCCD/600-7/2/107) and methods were executed in accordance with the approved guidelines and regulations. Coral cores were imported under CITES licence number 002259. The two reefs, Eve's Garden (EG) and Anemone's Garden (AG), lie along an inshore to offshore gradient and at different depth, distance from shore and distance from river mouths ( Fig. 1; Table 1) 45 . Coral cores were drilled using a SCUBA tank driven pneumatic drill (Silverline Air Drill Reversible), with a diamond-coated drilling head. Core segments of approximately 30 cm in length and 4 cm in diameter were obtained along the central growth axis of massive, dome-shaped Porites sp. colonies. Cores were then sectioned into 7-8 mm thick slabs longitudinally, along the axis of growth and cleaned following the chemical treatment method developed by Nagtegaal et al. 75 . Slabs were submerged for 24 hours in a bath of sodium hypochlorite solution NaOCl (with 6-14% active chlorine) diluted to a 1:1 ratio with distilled water. Thereafter, slabs were rinsed three times in an ultrasonic bath filled with distilled water for 10 min, with water exchanged after every run. In between turns coral segments were blown with compressed air to remove stray particles. Finally, slabs were dried in a drying cabinet at 50 °C for 24 to 48 h. X-ray radiography was used to visualize annual density bands and to determine the sampling path along the major growth axis (see Supplementary Fig. S4). Samples for geochemical analysis were drilled every 1.1 mm, using a diamond coated dental drill. The number of samples was n = 271 for Anemone's Garden and n = 103 for Eve's Garden. The sample powder was then split for stable isotope and Sr/Ca measurements.

Sr/Ca measurements and SST reconstruction.
Sr/Ca ratios were measured at the University of Kiel with a simultaneous inductively coupled plasma optical emission spectrometer (ICP-OES, Spectro Ciros CCD SOP), following a combination of the techniques described by Schrag 76 and de Velliers 77 . Sr and Ca were measured at their 421 and 317 nm emission lines, respectively. 175 ± 25 µg of coral powder was dissolved in 1 ml nitric acid (HNO 3 2%). Prior to analysis, this solution was further diluted with 4 ml HNO 3 2% to a final concentration of approximately 8 ppm. An analogously prepared in house standard (Mayotte coral) was measured after each sample batch of 6 samples to correct for drift effects. The international reference material JCp-1 (coral powder) was analysed at the beginning and end of every measurement run. Internal analytical precision based on replicate Sr/Ca measurements was 0.008 mmol/mol (1σ) or 0.08%. Average Sr/Ca value of the JCp-1 standard from multiple measurements on the same day and on consecutive days was 8.831 mmol/mol with 0.085% relative standard deviation (RSD). The comparison to the certified Sr/Ca value of 8.838 mmol/mol 78 with an expanded uncertainty of 0.089 mmol/mol indicates a high external precision of <0.08%.
SST reconstructions in this study are based on calibrations of the coral Sr/Ca ratios with the OISSTv2 data. The relationship between the skeletal Sr/Ca and the instrumental data was quantified by (OLS) regression 79 .
The error estimate for absolute SST reconstructions was calculated with the mean squares of the residuals of the inverted Eq. 1 41 .
The calculation of Sr/Ca-SST anomalies is based on relative changes in SST in comparison to a climatological mean. This method was used to eliminate errors associated with absolute SST reconstructions, since SST anomalies are independent from the intercept of the calibration. Sr/Ca anomalies for the AG record were calculated relative to the 1983 to 2015 mean, and converted to SST based on a range of published slopes to indicate uncertainties of the estimate 26,29 . Anomalies for the shorter records starting in 2006 of AG and EG were calculated relative to the average of 2006 to 2015. Calculation of anomalies was carried out with the KNMI Climate Explorer 80 . Based on empirical studies by Corrège 26 and Gagan et al. 29 , the slopes of −0.06 mmol/mol °C −1 and −0.084 mmol/ mol °C −1 were used for the conversion of Sr/Ca anomalies to SST next to local calibration slopes.
Oxygen isotope measurements. Stable oxygen and carbon isotope measurments were performed at the Museum für Naturkunde, Leibniz-Institut für Evolutions-und Biodiversitätsforschung, Berlin (Germany). 200 ± 50 µg of coral powder from each sample was put into clean 10 ml exetainers. After being sealed with a septum cap, the remaining air was flushed out of the vessel with helium gas for 6 min at a flow of 100 ml/min. This process was followed by an injection of 30 µl of anhydrous phosphoric acid (H 3 PO 4 ) through the septum. Each sample had a reaction time of approximately 90 min at 50 °C before measurement. Oxygen and carbon isotopic composition of the reaction product (CO 2 ) was measured using a Thermo Finnigan GASBENCH II coupled with a Thermo Finnigan DELTA V isotope-ratio mass spectrometer (IRMS). Pure CO 2 was used as reference gas for the measurements. The reference gas was calibrated against the VPDB (Vienna Pee Dee Belemnite) standard by using the IAEA (International Atomic Energy Agency) standards NBS18 and NBS19. Isotope values are shown in the conventional delta notation (δ 18 O) in per mil (‰) versus VPDB. The internal analytical precision based on replicate measurements of the reference gas was <0.05‰. The reproducibility of replicated lab standards (pulverized limestone) was generally <0.1‰ (1σ) for both carbon and oxygen isotopes.
Age model. Age-depth models for each core were established using the software AnalySeries2.0 81 and the NOAA AVHRR-OISSTv2 High Resolution Dataset for a 0.25° grid 46,47 . The chronology was generated based on the pronounced seasonal cycle of Sr/Ca ratios. The highest Sr/Ca values of each cycle were assigned to the coldest month from the instrumental data for each year. In cases of clear low peaks in the Sr/Ca ratios, these were assigned to the warmest months. The starting year was assigned to 2016, since the cores were drilled from living colonies. First, age assessments for all samples between the anchor points were obtained by linear interpolation in AnalySeries2.0 81 . In a second step, the sample data were interpolated to 12 equidistant points per year, creating a monthly time series. The time scale error in any given year is 1 to 2 months, due to interannual differences in the exact timing of peak SST. As such, AG covers the period 1982 to 2016 while EG covers 2006 to 2016. Application of lagged correlations to monthly/seasonal climate indices (Fig. 6) revealed the highest correlations at zero lag, suggesting that our linearly interpolated age model did not obfuscate seasonal trends in geochemical proxy data, despite the 1-2-month uncertainty in identifying annual anchor points. δ 18 o sw reconstruction. For the calculation of δ 18 O sw the method of Ren et al. 44 was followed, with the assumption that coral Sr/Ca is solely a function of SST and that coral δ 18 O is a function of both SST and oxygen isotopic composition of the seawater. The method uses instantaneous changes of both variables instead of looking at the absolute values. Effects of seawater isotopy on coral δ 18 O are separated from thermal effects by breaking the instantaneous changes of coral δ 18 O into separate contributions by instantaneous SST and δ 18 O sw changes, respectively. This is possible due to paired measurements of Sr/Ca and δ 18 O on the coral. In these calculations, we used a slope of −0.2‰ per 1 °C 82 for δ 18 O coral -SST, and for Sr/Ca-SST a slope of −0.06 mmol/mol °C −1 26 . Error estimation was performed by neglecting the error caused by non-climatic factors that may influence the proxies. The error σ δ18Osw in this study is 0.103‰. It was calculated following the approach of Cahyarini et al. 38 (Eq. 2), with σ δc being the error of measured δ 18 O coral , σ Sr/Ca being the error of measured Sr/Ca coral , and γ 1 and β 1 being the slopes of the linear regression of δ 18  Historical climate data and climate indices. Local mean air temperature data from the Miri airport station (WMO station No. 964490, 4.4°N, 114.0°E, elevation: 51 m a.s.l.) were obtained from the Global Historical Climatology Network-Monthly (GHCN-M version 3) temperature quality-controlled dataset 83 . Data were provided by the National Centers for Environmental Information (NCEI) of the U.S. National Oceanic and Atmospheric Administration (NOAA), downloaded from KNMI Climate Explorer 80 . Local monthly precipitation data from the Miri airport station (WMO station No. 964490, 4.4°N, 114.0 °E, elevation: 51 m a.s.l) were obtained from the Global Historical Climatology Network Monthly (GHCN-M version 2) quality-controlled dataset, which contains temperature, precipitation, and pressure data 84 . Data were provided by NOAA's NCEI, downloaded from KNMI Climate Explorer 80 .
River discharge data for the Baram River catchment were provided by the Department of Irrigation and Drainage (DID) in Malaysia 15 . The nearest station to the Baram outflow north of Miri was Marudi station (4.10°N, 114.18°E), which provided continuous data coverage between 1989 and 2015.
The NOAA 0.25° daily Optimum Interpolation Sea Surface Temperature version 2 (OISSTv2) dataset was used 46,47 . The dataset combines observations from ships, buoys and satellites (infrared satellite data from the Advanced Very High Resolution Radiometer AVHRR), with data gaps filled by interpolation. Daily SST data was available at a 0.25° grid resolution. The grid used in this study was 4.24-4.5°N, 113.75-114.00°E. Data were provided by NOAA's NCEI, downloaded from KNMI Climate Explorer 80 .